Electrohydrodynamic spraying has been used to process liquids into structures having sizes on the micrometer and nanometer scale. An electrohydrodynamic spraying apparatus applies a charging voltage to a liquid, resulting in an accumulation of repulsive electrostatic force within the liquid. When the repulsive electrostatic force exceeds the surface tension force, the surface of the liquid is disrupted to form small jets of liquid. These small jets then break up into streams of charged liquid clusters, which are referred to as “nanodrops” when the dimensions of the clusters are on the order of 100 nanometers (nm) or less.
Typically, nanodrops produced by electrohydrodynamic spraying are directed to the surface of a substrate material, which may be neutral or which may have an electric charge opposite that of the drops. If sufficient numbers of nanodrops accumulate on the substrate, the nanodrops will tend to coalesce and form a thin film. Nanodrops containing reactive material can be subjected to reaction conditions such that the nanodrops are converted into nanoparticles. Nanodrops also may be converted into nanoparticles by directing the nanodrops into a flask containing an appropriate liquid. For example, nanodrops containing a polymer can be converted into nanoparticles if the liquid in the flask is a nonsolvent for the polymer.
A specific example of electrohydrodynamic spraying is the Charged Liquid Cluster Beam (CLCB) technique. In CLCB, the electrostatic charge is injected into the liquid by a sharp, high-voltage electrode immersed in the liquid, where the liquid flows past the electrode and through a spray nozzle. The resulting nanodrops can then be directed to a substrate material. The size of the nanodrops is strongly dependent on the voltage applied to the electrode and on the flow rate of the liquid past the electrode. Modification of temperature gradients between the liquid and the spray nozzle and between the spray nozzle and the substrate can provide control over the final nanostructure formed on the substrate. Typical nanostructures include nanodrops, nanoparticles, and thin films. For thin film structures, all of these processing parameters also can be adjusted to control the morphology of the thin film, such as the size and shape of the film, the thickness of the film, and any variations or gradients in the thickness of the film.
Electrohydrodynamic spraying techniques, including CLCB, typically have been limited to use with substrates having a surface area less than 10 square centimeters (cm2). The electrostatic repulsion between the liquid jets tends to configure the spray from the nozzle in the shape of a cone. If the target surface area is too large and/or if the distance between the spray nozzle and the substrate is too great, the spray cone will tend to spread out and form a ring on the substrate. Electrostatic repulsion between nanodrops formed from an individual liquid jet can further contribute to the non-uniformity of the film, leading to an overall morphology of a ring made up of circular patches of nanodrops. In addition to limiting the sizes of films produced, these disadvantages can also hinder the adjustment of an electrohydrodynamic spraying apparatus to accommodate different materials or applications. For example, the distance between the spray nozzle and the substrate cannot be changed without affecting the morphology of the deposited nanodrops and the resulting thin film.
It is thus desirable to provide an electrohydrodynamic spraying system that can deposit a uniform thin film onto a substrate over a relatively large area. It is also desirable that such a system would be capable of adjustment so as to provide films having varying morphologies.